In order to meet the demand for increased driving comfort and performance demanded of a vehicle by sporty drivers through the control of the transmission, exact knowledge of the behavior of the system to be controlled is necessary. This is important in particular for the realization of functions by which demanded operating state changes of automatic transmissions, such as dual-clutch transmissions, are to be performed within short operating times. During the execution of such so-called timing-sensitive functions, an accuracy of the timing of between 10 and 20 milliseconds must be ensured, for which reason deviations between a target system behavior and an actual system behavior must already be correspondingly taken into consideration through pilot control of the system in order to be able to achieve a required reproducibility of the system behavior for a driver of a vehicle equipped with an automatic transmission of said type.
In general, hydraulic controllers of automatic transmissions have, out of their working principle, a “dead time” in the response behavior. As a result the target specifications of a hydraulic system are implemented in the real system only after the expiry of defined operating times. The dead time of the response behavior of a hydraulic system of an automatic transmission varies to a not inconsiderable extent as a function of the operating temperature of the hydraulic fluid of the hydraulic system and also to a certain extent as a function of tolerances of pilot stages and geometrical and mechanical tolerances such as valve geometries, spring characteristic values and the like, and is thus specific to the individual parts. If the scatter owing to manufacturing tolerances is too great, this particularly limits the performance that is reproducibly attainable.
In the case of automatic transmissions known from practice, it is sought, for example, to reproduce the dead time behavior of clutch actuation paths with respect to the temperature of electronic transmission controllers with models, and to correspondingly readjust the scattering component with the aid of an observer function if necessary.
This purely reactive approach by the observer function is disadvantageous because it is able to take into consideration deviations of the system behavior resulting from a corresponding adaptation of the actuation during operation of a hydraulic system only during the execution of robust functions, because there is enough time available during the execution of such robust functions. To be able to ensure acceptable reproducibility even during the execution of performance-emphasized functions, the exact knowledge of the actuation timing of the individual transmission components is already necessary in advance. Then, the respective sequences or target specifications of the individual components can be exactly coordinated with one another, and both demanded driving comfort and corresponding performance is achievable.
Furthermore, the response behavior or a dead time of a hydraulic system of an automatic transmission also varies in a manner dependent on the extent to which air accumulations are present in the hydraulic actuation paths. Such air accumulations exist in clutch actuation paths particularly after relatively long interruptions in operation, during which the hydraulic supply to hydraulic systems of automatic transmissions is substantially equal to zero. During the operation of automatic transmissions, their hydraulic systems are normally deaerated as required toward an oil sump by means of constant leakage volume flows.
At present, the response behavior of the actuation path of clutches of automatic transmissions is taken into consideration by corresponding adaptive algorithms in the electronic transmission controller. Further parameters, such as characteristic curve deviations, scatter relating to the torque transfer capacity of clutches and stiffness transitions of the clutches are determined adaptively during operation. Residual inaccuracies that arise in a manner dependent on the operating state are compensated during the operation of an automatic transmission by corresponding controllers.
In order to be able to meet comfort demands and correct erroneous actuations despite the scatter of the operating behavior of the clutches, the system is given a relatively large amount of time. The times available for the regulation however cannot be reduced to any desired extent, and furthermore, the operating time required for the correcting action is not always available. Therefore, during operating state profiles or during specific driving profiles, for example during a sport mode or during a racetrack operating mode or the like during which the driver comfort demand is not a priority and the driver rather desires high performance, controlled sequences are provided for the operation of an automatic transmission. Through the sequences, demanded operating state changes such as, for example, performance shifts, are implemented within shorter operating times than operating state changes executed in regulated fashion.
The two clutches of a dual-clutch system of a dual-clutch transmission are actuated in the manner described in more detail below during a performance-emphasized ratio change in the dual-clutch transmission such that the clutch assigned to the transmission part in which the presently selected ratio is engaged is opened, whereas the further clutch assigned to the further transmission part in which the target ratio has been engaged in preparatory fashion is transferred in parallel thereto into the closed operating state. For this purpose, the clutch for engagement is transferred within short operating times toward its closed operating state by a fast-charging pulse while the further clutch of the dual-clutch system is in the closed operating state, wherein the actuating pressure of the clutch for engagement is at the end of a fast-charging phase reduced to a defined pressure level at which the clutch for engagement exhibits its desired torque transfer capability.
At the time point of the demand for the ratio change, a time period is determined in a manner dependent on the present operating state of the dual-clutch transmission, after the expiry of which time period the actual actuating pressure of the clutch for engagement will exceed a defined pressure threshold in a manner dependent on the specification of the target actuating pressure of the clutch for engagement, and the target actuating pressure of the clutch for disengagement is abruptly reduced to the opening pressure level in order to transfer the clutch for disengagement into its open operating state to an extent coordinated by the actuation of the clutch for engagement.
Furthermore, the target actuating pressure of the clutch for disengagement is reduced at a time point from the pressure level of the fast-charging phase or from the fast-charging pressure level in a defined manner to the closing pressure level, when the clutch for engagement fully transmits the acting torque. The demanded performance shift is then executed as desired if the clutch for disengagement then substantially no longer transmits any torque at said time point.
By the system parameters likewise adapted as described above, it is possible to calculate a suitably exact and relatively short lengthening of the fast-charging pulse and to thus increase the actual pressure of the clutch for engagement directly with maximum system performance to virtually any target transmission level by a targeted overcharging of the piston chamber of the clutch for engagement, wherein the maximum system performance is limited by the hydraulic delay time or by the dead time of the hydraulic system.
To be able to execute a performance shift with high shift quality within a short operating time, the clutch for disengagement must, as required, be adjusted toward its fully open operating state at the correct time point as abruptly as possible and before the time of taking-on of load by the clutch for disengagement.
Since a blending phase of the overlapping shift without an interruption of tractive force between the two clutches is not defined by targeted control and regulation, as in the case of conventional and more comfort-emphasized shifts, but rather is defined only by the hydraulic delay time of the hydraulic system, the timing relating to the actuation of the two clutches must be defined very accurately in order to ensure or set the correct torque balance during the blending phase.
If the actuation timing of the clutches is not realized owing to corresponding inaccuracies in the system behavior in certain operating ranges of the dual-clutch transmission or is disrupted as a result of states such as air inclusions in the actuation path of the clutches of the hydraulic system, then the performance shift to be executed is likewise impaired owing to a disrupted torque balance during the blending phase of the two clutches of the dual-clutch system, which gives rise to discontinuities in the profile of an output torque, which impair driving comfort.
If the actuating pressures of the clutch for engagement and of the clutch for disengagement cross too early, a transmission deficit occurs during a performance shift, and thus an undesired load-release shock occurs during a traction upshift. In contrast to this, the two transmission halves of a dual-clutch transmission are braced relative to one another if the actuating pressures of the two clutches of the dual-clutch system cross too late.
The accuracy for the timing required for the execution of a performance shift lies in the range from approximately 10 to 20 milliseconds owing to the high performance demands as discussed above. In order to be able to realize such an exact actuation of the clutches, highly accurate knowledge of the system parameters under the present operating conditions, such as the system pressure, the pump volumetric flow, the operating temperature, the drive rotational speed and the like, is required.
The abovementioned adaptation algorithms are however directed out of principle to the determination of the reproducible component specific to the individual parts under steady-state conditions. If a corresponding component is not detected by the adaptation algorithm or if a sporadically occurring scatter is too great, for example owing to air inclusions in the clutch actuation path or the variance of the operating conditions, the performance that is reproducibly attainable with acceptable comfort is limited.
This situation is countered in practice through the fact that sensitive transmission functions such as the performance shift are permitted only in limited operating ranges and only during extremely sporty driving programs activated by the driver, because a driver then tends to accept impairments in comfort.
Present efforts to reduce the energy consumption of motor vehicles are supported inter alia through efficiency improvements of automatic transmissions. For this purpose, it is attempted inter alia to reduce leakage volume flows of hydraulic systems to a minimum, which however impairs a continuous deaeration of the hydraulic system by the leakage volume flows and has the effect that the hydraulic systems are often not sufficiently deaerated, and as a result the respective desired functional behavior is not ensured. Such a sufficiently deaerated or ensured operating state of the hydraulic system of an automatic transmission often cannot be determined as required over wide operating ranges of an automatic transmission or of a vehicle power train equipped therewith, as a result of which timing-sensitive and performance-emphasized functions are prevented in the absence of knowledge of such an operating state of the hydraulic system in order to avoid undefined operating states of an automatic transmission.
It is duly possible for functional impairments during the operation of an automatic transmission which result from air inclusions in the hydraulic system to be compensated by corresponding countermeasures if sufficient control and regulating times are available. This is however not suitable for timing-sensitive and performance-emphasized processes for which such countermeasures must be implemented with the correct timing, because such an observer or regulator provides its feedback too late and is therefore not usable. Here, it is particularly critical that, owing to a multiplicity of intercoordinated processes, timing-sensitive actuation sequences can no longer be stopped or terminated after having been started in order to avoid reactions in the vehicle power train which impair driving comfort, if necessary.
Regardless of possible air inclusions in hydraulic systems of automatic transmissions, the response behavior of automatic transmissions varies in each case in a manner dependent on the respectively present operating point and on component-specific tolerances and in a manner dependent on manufacturing tolerances, for which reason each automatic transmission or each hydraulic system has, even in the fully deaerated operating state, a reproducible component of a dead time or of the response behavior which is specific to each individual example.
Since automatic transmissions are operated with an applicatively determined and averaged dead time value, in the case of automatic transmissions whose reproducible dead time specific to each individual example is greater than the average applicative dead time, timing-sensitive and performance-emphasized functions are prevented despite a hydraulic system of an automatic transmission being in a fully deaerated operating state. Furthermore, in the case of transmissions whose reproducible dead time specific to each individual example lies below the averaged applicative dead time, even in the non-fully-deaerated operating state of its hydraulic system, a reliable response behavior is determined and timing-sensitive and performance-emphasized functions are started and executed despite air inclusions in the system, whereby driving comfort is, however, impaired in an undesired manner.